Mechanical pump jacks have been used for many years in the oil and gas industry to remove liquids from deep wells. Typically, a rocking beam is connected at one end to a string of sucker rods which actuates a downhole pump mechanism and is counterbalanced with heavy weights at the other end to reduce the uplift force required to raise the sucker rod and liquids contained in the well.
One of the drawbacks of this arrangement is that the sucker rod string follows a generally nonadjustable sinusoidal velocity profile. Certain well applications may be limited by a maximum permissible upstroke and/or downstroke velocity. When coupled with the fixed sinusoidal motion of the rocking beam, the velocity limitation constrains the overall stroke rate, and therefore the overall well production rate.
Furthermore, the loads generated by the dynamics of the system may dictate that the well is best operated according to some profile other than the generally nonadjustable sinusoidal profile. The overall efficiency of the system and component life may be improved by reciprocating the well according to an alternate velocity profile. A system which permits adjustment of the stroke velocity profile would be very desirable.
Hydraulic systems, which permit a greater degree of control of the velocity of the sucker rod string, are known. In general, these systems utilize a secondary cylinder or pressure area to assist the primary cylinder and provide counterbalance. Since the upstroke and downstroke forces are in the same direction, some of the energy put into the system on the upstroke may be recovered, through the use of counterbalance, on the downstroke. However, the addition of another cylinder to the system reduces reliability, often increases overall height, and increases system complexity. A hydraulic unit that provides a means for counterbalance without the addition of a second cylinder would create a simpler, more space efficient, and inherently more reliable machine.
An additional shortcoming of both existing prior art hydraulic and mechanical systems is that no means are provided for diagnosing the development of problems downhole. For example, failure of the pump, leakage in the pump, changes in the liquid makeup in the well, dry bottom conditions in the well, excessive sucker rod drag, will all manifest themselves by changes over time in the work increments being done by the unit. A system to track these increments to permit diagnosis of problems downhole would be very desirable. Also, a system to counteract leakage in the pump by adding hydraulic fluid makeup on an as needed and controlled basis would be very desirable, as some leakage in the pump is inherent.
Pump jacks do not require a derrick for operability. A derrick is required for a hydraulic actuator for sucker rod. A derrick which is easy to transport and assemble and is inexpensive would be very desirable for use with hydraulic sucker rod actuator systems.